Image capturing apparatuses such as image scanners, facsimile machines, printers or copiers are widely used for capturing or scanning images of documents, photographs or films. As known, the image capturing apparatus usually has a document feeder for successively and continuously feeding many paper sheets. Typically, the document feeder has a sheet pick-up device in contact with the front edges of the documents so as to feed the paper sheets one by one.
Referring to FIGS. 1A and 1B, a schematic cross-sectional view of a sheet pick-up device of a document feeder is illustrated. FIG. 1A schematically illustrates the sheet pick-up device in a feeding mode. FIG. 1B schematically illustrates the sheet pick-up device in a ready mode. The sheet pick-up device 2 is disposed under an upper cover 3. The operation principles of sheet pick-up device in the feeding mode and the ready mode are illustrated as follows.
As shown in FIG. 1, when the document feeder 1 is operated in the feeding mode, a driving shaft 21 of the sheet pick-up device 2 is driven by a motor (not shown) to rotate in a direction R1, and a sheet pick-up arm 22 of the sheet pick-up device 2 is descended to a sheet feeding position. Meanwhile, a sheet pick-up roller 221 of the sheet pick-up arm 22 transports a document S forwardly and a sheet separation roller 222 of the sheet pick-up arm 22 is rotated to separate the top paper sheet from the stack of paper sheets, thereby picking a single paper sheet.
As shown in FIG. 1B, after the document S has been fed across the sheet separation roller 222, the driving shaft 21 is driven by the motor to rotate in a direction R2, the sheet pick-up arm 22 of the sheet pick-up device 2 rises up to a ready position.
The above document feeder 1 has several drawbacks. For example, when the driving shaft 21 is rotated in a direction R2 to raise the sheet pick-up arm 22 to the ready position, the motor is not instantly switched off. Since the driving shaft 21 is continuously driven by the motor, the sheet pick-up arm 22 may be further raised to strike the upper cover 3. Under this circumstance, undesirable noisy sounds are emitted and even the upper cover 3 is uplifted.
For avoiding generation of noisy sounds when the sheet pick-up arm 22 strikes the upper cover 3, the sheet pick-up device 2 should be improved. Referring to FIG. 1C, a schematic perspective view of the sheet pick-up device 2 is illustrated. The sheet pick-up device 2 of FIG. 1C is advantageous for reducing the possibility of striking the upper cover 3 and/or uplifting the upper cover 3 by the sheet pick-up arm 22. As shown in FIG. 1C, a clutching member 23 including two one-way spring clutches 23A and 23B is mounted on the driving shaft 21. When a sheet feeding operation of the document feeder 1 is initiated, the spring clutch part 23A is driven to descend the sheet pick-up arm 22. When the sheet feeding operation is terminated, the spring clutch part 23A is driven to raise the sheet pick-up arm 22 to the ready position. Next, when the sheet pick-up arm 22 is further raised to touch the upper cover 3, the spring clutch part 23B is operated to allow for idle running of the driving shaft 21, thereby reducing the possibility of striking the upper cover 3 and/or uplifting the upper cover 3 by the sheet pick-up arm 22. A sheet pick-up device which has two one-way spring clutches was disclosed in for example U.S. Pat. No. 6,390,463, the contents of which are hereby incorporated by reference.
Although the approach of mounting two one-way spring clutches on the driving shaft is advantageous for reducing the possibility of striking the upper cover and/or uplifting the upper cover by the sheet pick-up arm, there are still some drawbacks. For example, the extra one-way spring clutch increases cost and complexity of the sheet pick-up device.
In views of the above-described disadvantages resulted from the prior art, the applicant keeps on carving unflaggingly to develop an improved sheet pick-up device according to the present invention through wholehearted experience and research.